Measurements of sound speed in excised tissue over temperatures expected under high-intensity focused ultrasound conditions

Abstract

Noninvasive monitoring of high-intensity focused ultrasound (HIFU) treatment by diagnostic ultrasound is currently limited by a poor understanding of the bioacoustic effects of tissue heating. Sound-speed measurement has been proposed as a means of noninvasive thermal mapping. The sound speed in most tissues changes monotonically but nonlinearly with temperature to around 60 °C. Above that temperature, at which the tissue proteins begin to denature, irreversible changes occur in the tissue and its corresponding acoustic properties. Sound-speed data around and above the range in which the tissue ‘‘cooks’’ is a challenge to obtain because of changes in tissue shape, compressibility, and overall dimensions associated with the cooking process. In this research, an apparatus to measure sound speed during a controlled temperature change was used to study the effects of temperature, thermal dose, and thermal dose rate on the speed of sound in various mammalian tissues. The apparatus, potential sources of error, and results will be presented, and implications for forward and inverse HIFU propagation modeling and acoustic remote sensing of HIFU-induced temperature changes will be discussed. [Work supported by DARPA.]